docs: def Kawahara(x, rhol, rhog, D):
      r'''Calculates void fraction in two-phase flow according to the model
      presented in [1]_, also reviewed in [2]_ and [3]_. This expression is for
      microchannels.
  
      .. math::
          \alpha = \frac{C_1 \alpha_h^{0.5}}{1 - C_2\alpha_h^{0.5}}
          
      Parameters
      ----------
      x : float
          Quality at the specific tube interval []
      rhol : float
          Density of the liquid [kg/m^3]
      rhog : float
          Density of the gas [kg/m^3]
      D : float
          Diameter of the channel, [m]
  
      Returns
      -------
      alpha : float
          Void fraction (area of gas / total area of channel), [-]
  
      Notes
      -----
      C1 and C2 were constants for different diameters. Only diameters of
      100 and 50 mircometers were studied in [1]_. Here, the coefficients are
      distributed for three ranges, > 250 micrometers, 250-75 micrometers, and
      < 75 micrometers.
      
      The `Armand` model is used for the first, C1 and C2 are 0.03 and 
      0.97 for the second, and C1 and C2 are 0.02 and 0.98 for the third.
  
      Examples
      --------
      >>> Kawahara(.4, 800, 2.5, 100E-6)
      0.9276148194410238
  
      References
      ----------
      .. [1] Kawahara, A., M. Sadatomi, K. Okayama, M. Kawaji, and P. M.-Y. 
         Chung. "Effects of Channel Diameter and Liquid Properties on Void 
         Fraction in Adiabatic Two-Phase Flow Through Microchannels." Heat 
         Transfer Engineering 26, no. 3 (February 16, 2005): 13-19. 
         doi:10.1080/01457630590907158.
      .. [2] Xu, Yu, and Xiande Fang. "Correlations of Void Fraction for Two-
         Phase Refrigerant Flow in Pipes." Applied Thermal Engineering 64, no. 
         1-2 (March 2014): 242–51. doi:10.1016/j.applthermaleng.2013.12.032. 
      .. [3] Dalkilic, A. S., S. Laohalertdecha, and S. Wongwises. "Effect of 
         Void Fraction Models on the Two-Phase Friction Factor of R134a during 
         Condensation in Vertical Downward Flow in a Smooth Tube." International 
         Communications in Heat and Mass Transfer 35, no. 8 (October 2008): 
         921-27. doi:10.1016/j.icheatmasstransfer.2008.04.001.
      '''
      if D > 250E-6:
          return Armand(x, rhol, rhog)
      elif D > 75E-6:
          C1, C2 = 0.03, 0.97
      else:
          C1, C2 = 0.02, 0.98
      alpha_h = homogeneous(x, rhol, rhog)
      return C1*alpha_h**0.5/(1. - C2*alpha_h**0.5)